Some researchers will "believe" something when they have three sources that provide the same piece of information. One has to be careful using this approach. Sources may all contain information from the same person or "original source," which does not really mean that three "sources" agree. It could only mean that the same person gave the information three times.
And there is always the chance that the second two "sources" got their information from the first.
Think about who provided the information, why it is in the record, and how reasonably the informant would have known the information. That's a good way to get started with information analysis.